Hydropower generating systems, i.e., hydro-turbines, are dependent on a continuous source of water, generally drawn from an open source, such as an impoundment, river or lake. Water intake requirements amount to thousands of gallons per minute. Turbines used to transform the associated hydrostatic head into electric power are designed with cascades, each comprising a fixed stay vane and an adjustable valve, termed a wicket gate. The cascade, most often arranged in a circle about the turbine runners, directs the water flow into the turbine runner at the desired angle to optimize energy recovery. These cascades are placed in the penstock tube, i.e., the conduit used to transport water to the turbine. Typically there are 20 to 24 cascades for a large turbine. The cascades direct water to the runners of the turbine where the flow is converted to rotary turbine motion. The cascades provide regulation of the turbine by directing water into the turbine at a flow orientation and flow rate to meet a pre-specified demand. This regulation is achieved by adjusting the openings of the cascade to compensate as the hydrostatic head varies with changes in the environment. Further, the cascade modulates sudden flow surges when the turbine starts and stops.
These turbines are often provided as reversible pump-turbines, i.e., water motion may be converted to mechanical motion or vice versa. That is, the cascades are also capable of regulating the flow of water from the turbine runner back through the penstock tube. This is necessary when a sudden loss of power to the generator occurs and back-flow must be prevented.
During operation, the demand for water may vary between a maximum possible intake, to nearly no water within a short period. At high water flow rates the cascades may be plagued by erosion and pitting from cavitation. At low or no water flow, marine growth and corrosion result. These problems directly alter the surface profile of the cascades, resulting in a reduction in the effective water flow rate through the penstock tube to the turbine runners. Cascades are designed with shapes that permit optimized water flow with minimal turbulence. Because the wicket gates move and the stay vanes are fixed, conventionally, more attention is paid to addressing changes to the design of the wicket gates than the stay vanes. See, for example, U.S. Pat. No. 5,228,830, Wicket Gate, to Pastore, Jul. 20, 1993; U.S. Pat. No. 5,441,384, Hydraulic Turbine and Guide Gate Apparatus and Runner Apparatus Therefor, to Gokhaman, Aug. 15, 1995; and U.S. Pat. No. 4,203,703, Hydraulic Turbo Machine Wicket Gate Seals, to Koeller, May 20, 1980.
Erosion is the gradual wearing-away of a surface. Impinging particles carried along by rapidly moving water causes it. Pitting is caused by cavitation. Pitting occurs when gas-filled cavities, as may be created by structure vibration, collapse on the surface. This collapse imparts a pressure pulse of such magnitude that it removes small pieces of the surface. A pitted or eroded surface sets up turbulence and associated vibration, thereby accelerating damage. Means to prevent both corrosion and pitting include optimizing water flow to minimize particles impinging on surfaces of the cascade by improving the orientation of flow to reduce turbulence in the area of the entrance to the runners.
To optimize operation of a turbine, regular maintenance is done on the cascades. Due to their location, it is difficult to access them, requiring shutting down and sending divers into the penstock tube. The divers inspect, and, if possible, make repairs in situ. When in situ repairs are impossible, the wicket gates may be removed and brought to the water surface. Further, due to the dimensions and weight of the gates, they must be removed individually. The stay vanes are part of the structure, however, and require careful attention to remove and replace only some of them at a time. Because of the awkwardness of the retrieval operation, the gates or stay vanes are dragged through the tube, often resulting in damage to the removed structures, the penstock tube, or both. Thus, it is important, both operationally and fiscally, to minimize damage to the cascades and extend maintenance intervals wherever possible.
An approach to minimizing the effects of cavitation is through “hydro-dynamically shaping” turbine components, including the cascades. This increases the maximum flow rates while minimizing turbulence at the boundary between the turbine parts and the flowing water. As discussed above, the majority of effort has been to address the shape of only the wicket gates in conventional efforts at improving flow. Although these efforts have ameliorated the effects of erosion and cavitation, they have not eliminated them. Thus, periodic maintenance of the cascade components is still required. Design optimization techniques have included selection of the location of the turbine and control of the turbine operation. Changes in these parameters may result in a reduction in the effects of erosion and cavitation, but such a reduction often comes at the expense of the power-generating capacity of the hydroelectric unit. The net effect of such an effort is an extension of the maintenance lifecycle at the cost of a reduction in power output. An embodiment of the present invention not only has the potential for increasing required maintenance intervals but also increasing turbine output while reducing fauna (fish) casualties.
A conventional turbine-machinery stay vane is separated from its respective wicket gate by a “generous” clearance gap. Mainly due to hydraulic shear and cross flow between stay vane and wicket gate, waterborne fauna may be severely injured or killed when passing through the turbine. Thus, an embodiment of the present invention also has the potential for reducing fauna (fish) casualties.
Conventional design practice does not identify the size of this clearance gap. The immobile stay vanes are structural members used to hold the superstructure in place and transfer the building and operating loads to the main civil structure. Thus, it is advantageous to provide these structural members with a provision for replacement of sections of the structure (stay vane) that are susceptible to wear or amenable to modification due to an improvement in other components of the turbine, such as the wicket gates or turbine runners. A wicket gate is the removable and movable component of the cascade, thus it is not as important to provide separate removable pieces for it. The wicket gate permits real time adjustment of the cascade to meet operating requirements, whereas the design of the stay vane must be judiciously selected to “cooperate” with the other components of the turbine. This “cooperation” is based on the projected entire operating cycle of the turbine, including an option to be altered readily to “cooperate” with modifications to other components of that turbine over its projected life. Basic machine parameters, such as overall civil constraints, head, power and flow, determine the radial location, number and size of the wicket gates and stay vanes. An embodiment of the present invention addresses the option to alter stay vanes even though they are part of supporting structure.